This invention relates to data communication over telephone lines utilizing tone signals.
Pushbutton telephones use Dual Tone Multi-Frequency (DTMF) signals for transmission of numbers. Each button on the telephone generates a low and a high frequency signal corresponding to the row and column, respectively, in which that particular button is located on the telephone. The resulting DTMF signal is a composite signal consisting of a high frequency tone and a low frequency tone. The presently used frequencies are 697 Hz, 770 Hz, 852 Hz and 941 Hz for the low-frequency group and 1209 Hz, 1336 Hz and 1477 Hz for the high-frequency group. There is room for expansion to eight frequencies by adding another column.
Such a pushbutton telephone can be used to input data to a computer as shown by James et al., U.S. Pat. No. 3,647,973. As recognized by James, a single tone can be generated by depressing two buttons in any row or any column. This feature was an accidental by-product of the original design of the pushbutton phone, which had been deliberately copied in later phones. Basically, a single transistor is coupled to two LC circuits, one for the rows and one for the columns. The capacitors are coupled to different points on the inductor when a button is depressed. The depression of two rows (or two columns) results in the transistor not operating within its precisely defined linear range, and no row frequency (or column frequency) is produced. Thus, by pressing two buttons in the first row, for example, the tone 697 Hz will be produced. The James method uses such single tones for control signals and the standard DTMF tones for data signals. The use of the single tones thus adds to the complexity of the DTMF receiver used.
DTMF decoding requires the detection of two superimposed frequencies and thus requires sophisticated filtering. A simplified method of decoding DTMF signals involves using high and low band pass filters to separate the high frequencies from the low frequencies. In each of the separate channels thus generated, the zero crossings of the received signal are counted and compared to zero crossing counts corresponding to the respective frequencies. However, such a system still requires high and low band pass filtering. Examples of such a system, which are typically used for inputting signals to a computer, are shown in the following patents:
Laoteppitaks, U.S. Pat. No. 4,016,370 PA1 Schartmann, U.S. Pat. No. 3,790,720 PA1 Richards, U.S. Pat. No. 4,042,789 PA1 Friend, U.S. Pat. No. 3,537,001 PA1 Ball, U.S. Pat. No. 3,935,395 PA1 Ball, U.S. Pat. No. 3,949,177 PA1 Ohl, U.S. Pat. No. 3,971,897
DTMF receivers are typically not used with ordinary home telephones because of their complexity and corresponding high cost. Thus, devices which can be coupled to an ordinary home telephone, such as an answering machine, are typically not remotely controllable from a pushbutton phone. A few devices use a DTMF receiver with a simplified design to reduce the cost, with a resulting tradeoff in performance. The simple design limits the dynamic range and the tolerance to distortion and voice interference.
Most home answering machines have a receiver which can detect a single tone. This tone is generated by a portable tone generator which the user holds up to the telephone after dialing his home number from a remote location.
Due to the pervasiveness and standardization of DTMF pushbutton phones, the use of single tones for communication has been relatively ignored, while DTMF receivers have proliferated. The single tones which can be produced by a pushbutton phone have been used to increase the complexity of input data to a DTMF receiver for a computer as shown by James, above. For devices coupled to a home telephone, separate single-tone generators and receivers are commonly used. These single-tone receivers use only one tone which is unrelated to the single tones of a pushbutton telephone.